HMACT64 hmac = new HMACT64(TEST_KEY);
            byte[] result = hmac.engineDigest();

            assertArrayEquals(finalDigest, result);
            verify(mockMd5, times(1)).digest(); // First call for inner digest
            verify(mockMd5, times(2)).update(any(byte[].class)); // Once in constructor with ipad, once in engineDigest with opad
            verify(mockMd5, times(1)).digest(innerDigest); // Second call for final digest

    void testEngineDigest() {
        // Test engineDigest()
        HMACT64 hmac = new HMACT64(TEST_KEY);
        hmac.engineUpdate(TEST_DATA, 0, TEST_DATA.length);
        byte[] result = hmac.engineDigest();

        assertNotNull(result);
        assertEquals(16, result.length); // MD5 produces 16 bytes
    }

    @Test
    void testEngineDigestWithBuffer() {
        // Test engineDigest(byte[] buf, int offset, int len)

            throw new IllegalStateException(ex.getMessage());
        }
    }

    @Override
    protected byte[] engineDigest() {
        final byte[] digest = md5.digest();
        md5.update(opad);
        return md5.digest(digest);
    }

    @Override
    protected int engineDigest(final byte[] buf, final int offset, final int len) {
        final byte[] digest = md5.digest();
        md5.update(opad);

            throw new IllegalStateException(ex.getMessage());
        }
    }

    @Override
    protected byte[] engineDigest() {
        final byte[] digest = this.md5.digest();
        this.md5.update(this.opad);
        return this.md5.digest(digest);
    }

    @Override
    protected int engineDigest(final byte[] buf, final int offset, final int len) {
        final byte[] digest = this.md5.digest();

    }

    /**
     * Completes the hash computation by performing final operations such
     * as padding. At the return of this engineDigest, the MD engine is
     * reset.
     *
     * @return the array of bytes for the resulting hash value.
     */
    @Override
    public byte[] engineDigest() {
        // pad output to 56 mod 64; as RFC1320 puts it: congruent to 448 mod 512